Acoustic absorber, acoustic wall and method for design and production

1. An acoustic absorber device, notably passive absorber, comprising: an enclosure delimiting a cavity opening outwardly into an inlet direction through at least one orifice passing through a front wall of a determined thickness, thereby forming a neck having a determined opening surface and a determined length, the dimensions of said enclosure and said neck being determined to together form a Helmholtz resonator for a first frequency or frequency range, called natural frequency;

at least one mobile element, or wafer, is suspended to said enclosure by one or more mechanical connections, or suspensions, in a position partially obstructing said at least one neck, i.e. unsealed on all or part of its stroke; and

the stiffness of the suspensions and the stiffness of the wafer are determined in their combination, particularly in their ratio, so that said wafer vibrates in a “piston” type resonance mode along the direction of the incident wave, at a second frequency or frequency range different from the first frequency, particularly lower, thereby achieving absorption for this second frequency or frequency range.

2. The device according to claim 1 , characterised in that the wafer is made of one or more materials chosen from silicon, quartz, alumina, titanium and its alloys, steel, aluminium and its alloys, plastics and notably polymers.

3. The device according to claim 1 , characterised in that the suspensions are made of a material and using a geometry providing an elastic behaviour, with stiffness for the movement of the wafer in its periphery of less than 6 N/m, and in particular of less than 2 N/m for a wafer of average diameter between 10 mm and 20 mm.

4. The device according to claim 1 , characterised in that the suspensions comprise elongated arms connecting the wafer to the enclosure in a shape extending around said wafer parallel to the edge of the neck and/or the wafer.

5. The device according to claim 4 , characterised in that the wafer is made within a plate or a sheet integral with the enclosure, by a part rendered mobile with respect to said enclosure by means of one or more cutouts made in said plate or sheet so as to form suspension arms.

6. The device according to claim 1 , characterised in that the wafer is held in the neck by one or more advances protruding from the neck at both ends to extend in front of the periphery of the wafer so as to form a stop preventing said wafer from escaping from the neck.

7. The device according to claim 1 , characterised in that the wafer has a periphery which conforms to the inner surface of the neck with a determined deviation over a sufficiently determined length, in combination with said deviation and with the nature of the materials of the neck and the wafer, to allow said wafer to move along the neck without causing its blocking by tilting and arching.

8. The device according to claim 1 , characterised in that the wafer is formed by a diaphragm of speaker fixed to an outer frame by a flexible peripheral seal, and in that said seal has one or more cutouts surrounding said wafer over at least 20% of its periphery, and in particular at least 40%.

9. The device according to claim 1 , characterised in that the wafer further interacts with the enclosure by an electromagnetic system so as to form the membrane of a loudspeaker, and in that said electromagnetic system is controlled by an electronic circuit:

in order to achieve active acoustic absorption, and/or

so as to modify the acoustic impedance of said loudspeaker to enhance absorption, shift the absorption frequency, widen the absorption frequency range, or a combination of these effects.

10. A sound absorbing wall comprising a multitude of devices according to claim 1 , juxtaposed within a continuous two-dimensional array to provide acoustic absorption in a common direction.

11. The wall according to claim 10 , characterised in that it comprises a plate with a honeycomb structure whose housings form a multitude of cavities which are closed on a so-called rear side,

and whose cavities are covered on one front side by one or more walls cut to form a multitude of necks each receiving a wafer.

12. A process for the industrialisation of an acoustic absorber according to claim 1 , intended to absorb a target frequency, comprising:

a step of determining dimensions of a cavity provided with a neck so that said cavity and said neck form a Helmholtz cavity having a first frequency Helmholtz resonance higher than the target frequency; and

a step of determining characteristics of a suspended wafer adapted to be arranged in the neck of said cavity so as to produce an absorber tuned to a second frequency corresponding to said target frequency.

13. The method according to claim 12 , wherein the suspended wafer is determined so that the suspension of the absorber has its first normal mode of deformation at a frequency lower than the second frequency.

14. The method according to claim 13 , characterized in that the wafer of the acoustic absorber is determined so as to have, when it is free, its first normal mode of deformation at a frequency higher than the second frequency.

15. A method of manufacturing an absorber according to claim 1 , characterised in that it comprises at least one step of cutting out a sheet or plate so as to form one or more acoustic absorber wafers.

16. The method according to claim 15 , characterised in that the plate or sheet is fixed to the surface of a plate having a honeycomb structure, and in that the cutting out step produces a plurality of wafers distributed with respect to the housings of the honeycomb structure so as to form the plurality of wafer of an acoustic wall whose housings form a multitude of cavities which are closed on a so-called rear side, and whose cavities are covered on one front side by one or more walls cut to form a multitude of necks each receiving a wafer.